In the related art, exhaust gas purification systems including SCRs which selectively reduce and purify NOx in exhaust gas using ammonia (hereinafter, NH3) generated by hydrolysis from urea water as a reducing agent are known. In such exhaust gas purification systems, an amount of urea water being ejected is feedback-controlled so that a value of NOx in the exhaust gas reaches a target value based on a sensor value of an NOx sensor provided on an exhaustion downstream side of the SCR (for example, refer to Patent Literature 1 and Patent Literature 2).